In recent years, as clean energy, energy storage systems, and the like rapidly develop, so do DC power conversion systems. Because of large currents and high voltages in these systems, fault arcs are more likely to occur.
An arc is the phenomenon of electric discharge occurring when contacts in a conductor through which large currents pass or the contacts and another conductor are unreliably connected due to poor contact caused by unreliable connection between joints or insulation aging of a joint or a conducting wire. An arc is a major cause of fire in a DC system, and therefore whether occurrence of an arc is detected in time and accurately directly affects the security and reliability of the DC system.
Arcs include series arcs and parallel arcs. The former is arcs between cables and the latter is electric discharge from a DC end to a safety ground. The present invention mainly aims to resolve series arcs.
A photovoltaic power generation system is a DC application of high voltages and large currents. With the rapid growth of the photovoltaic industry, there are a growing number of mounted photovoltaic system devices. Security of photovoltaic systems is challenged by gradual aging of photovoltaic power generation devices put into operation and negligence during manual mounting. A DC side voltage of a photovoltaic system can reach up to hundreds of volts with the configuration of a photovoltaic panel. If a fault arc occurs, because there is no zero-crossing protection, a DC arc is more dangerous than an alternating current arc. Energy is generated when the photovoltaic panel of the photovoltaic system is continuously exposed in sunshine. If the fault arc that occurs is not perceived in time and a circuit is not cut off, while the panel provides outpouring energy to the arc, the power transmission circuit and the photovoltaic device are both greatly damaged. What's worse, fire is caused and life is threatened.
In view of safety and reliability, a growing number of DC applications propose requirements for detecting DC arcs. Some of the requirements have been forcibly required to be executed. For example, the 2011 version of US National Electrical Code (NEC) started to require to mount an arc fault detection circuit breaker on a photovoltaic power generation system. Requirements on DC fault arc detection in the NEC are specified in the UL1699B standard. The specification requires that an inverter should provide a visible alarm when detecting a fault arc and the alarm cannot be deleted automatically. The alarm should still be displayed when power is on again after a blackout, and the inverter can perform grid-tied working only after the alarm is manually deleted. An inverter needs to have a fault arc manual/automatic detection function, and the inverter needs to provide an alarm on a visible interface when performing automatic detection. If the automatic detection does not pass, the alarm needs to be manually deleted; otherwise, the alarm is saved and still exists when the inverter is restarted after a blackout. The UL1699B further specifies requirements on protection time, external environment, and the like.
On one hand, reliable and accurate arc detection is undoubtedly quite important to security of a DC system. On the other hand, how to prevent a detection unit from performing erroneous detection is also a difficulty during actual implementation. As required in the 2011 US NEC, after a detection unit gives an arc alarm, a photovoltaic inverter needs to stop power generation and is not allowed to continue to perform grid-tied power generation until an operator detects and manually deletes the alarm. Therefore, an erroneous detection may cause frequent shutdowns of a photovoltaic power generation system, affecting power generation benefit of the system.
Currently, common detection methods are as follows:
1. Determining Based on Radio Frequency Noises Generated by an Arc
In WO 95/25374, based on that radio frequency noises within a frequency band are generated after a DC current arc occurs, noise signals are received by using a radio frequency receiving apparatus, frequency division processing is performed, and noise amplitudes before and after an arc occurs are compared, to detect whether an arc occurs.
Because a power electronics device may generate noises, and electronic devices are more widely used, this solution cannot avoid erroneous detection. In addition, when a plurality of devices synchronously runs, a location where an arc occurs cannot be accurately determined or found.
2. Detection Based on Changes of Amplitudes of a DC Current High Frequency Component
National Instruments Corporation proposes to perform determining according to amplitudes of a DC current frequency spectrum.
The principle of this solution is also that high frequency noises may be caused to currents after a DC arc occurs. However, in this detection manner, DC currents are collected and FFT is performed on the currents, so that amplitude-frequency characteristics of the currents are obtained, and differences of amplitudes within a frequency band before and after an arc occurs are determined, thereby determining whether an arc occurs.
This solution is widely used because of relatively accurate detection and low costs. A disadvantage of the solution is also that the solution is subject to noise interference caused by a power electronics circuit and a surrounding device. When noise signals in the DC currents in the device are relatively large, this solution cannot be used or erroneous reports are frequently generated.